In recent years, the recording density achieved by magnetic recording has increased by 40% per year, due to developments in perpendicular magnetization media and adoption of a read head (TMR head) using a tunneling magneto-resistive (TMR) element as a magnetic sensor. As of 2009, a recording density of 500 Gb/in2 was achieved. In addition, as a measure for rapidly improving the recording density, patterned media are known.
Further, in recent years, a three-dimensional recording method utilizing a multilayered magnetic recording medium has been proposed as another measure for rapidly improving the recording density. In the three-dimensional recording method, a magnetic recording medium including a plurality of recording layers with different magnetic resonance frequencies is used, and selective writing data to the layers and selective reading data from them are performed by using a spin-torque oscillator and an auxiliary magnetic pole together.
In a conventional three-dimensional recording method in which reading of recorded data is performed by using one auxiliary magnetic pole and one spin-torque oscillator, a reading voltage is small and a polarity thereof does not depend on the direction of magnetization of a recording bit, and thus the bit error rate may increase.